$mw->after( 'idle', [
     configure => $styleref_select_parent, 
    -background => 'cyan', 
]);
[download]

#!/usr/bin/perl -l

use strict;
use warnings;

use Tk;
use Tk::Tree;
require Tk::ItemStyle;

my $mw   = new MainWindow;
my $tree = $mw->ScrlTree(
    -indicator => 1,
    -browsecmd => \&abba_browse,
)->pack( -fill => 'both', -expand => 1 );

my $styleref_select_parent = $tree->ItemStyle(
    'imagetext',
    -stylename  => 'stylename_select_parent',
    -foreground => 'dark blue',
    -background => 'cyan',
    -font       => [
        -family     => 'Linotype Birka',
        -size       => '-10',
        -weight     => 'bold',
        -slant      => 'roman',
        -underline  => 0,
        -overstrike => 0
    ]
);

my $styleref_select_child = $tree->ItemStyle(
    'imagetext',
    -stylename  => 'stylename_select_child',
    -foreground => 'red',
    -background => 'cyan',
    -font       => [
        -family     => 'Linotype Birka',
        -size       => '20',
        -weight     => 'bold',
        -slant      => 'roman',
        -underline  => 1,
        -overstrike => 1
    ],
);

$tree->add( 'ABC',        
     -text => 'Text is: ABC', 
     -state => 'normal',
);
$tree->add( 'ABC.def',    
     -text => 'Text is: def', 
     -state => 'normal', 
);
$tree->add( 'ABC.def.ghi', 
    -text => 'Text is: ghi', 
    -state => 'normal',
 );
$tree->setmode();
$mw->after( 'idle',
    [ configure => $styleref_select_parent, -background => 'cyan' ],
);

MainLoop;

sub abba_browse {
    my $tree_element = $tree->info('selection')->[0];

    if ( $tree_element eq 'ABC' ) {
        $tree->entryconfigure( $tree->info('selection'),
            -style => $styleref_select_parent );
    }
    else {
        $tree->entryconfigure( $tree->info('selection'),
            -style => $styleref_select_child );
    }

}
[download]